Drill string compensator

ABSTRACT

For use with a standard drilling rig on a floating barge, there is interposed between the travelling block frame and the drill string support a drill string compensator having a cylinder and piston expansible chamber (with or without a motion multiplier) furnished with air under pressure within a normally closed volume. The air chamber acts as a cushion and is arranged so that the variation in chamber volume produces only a small change in the air pressure, which can be set at any desired average value. Motion of the pneumatic, expansible chamber is controlled by a hydraulic expansible chamber. Various emergency and manual controls are provided.

United States Patent [191 Duncan et al.

[ Apr. 16, 1974 1 DRILL STRING COMPENSATOR [75] Inventors: Anthony Bruce Duncan, Piedmont,

Calif.; Richard Galen Sprague, Houston, Tex.; Arthur Hor Ting Chin, Piedmont, Calif.

[73] Assignee: The Rucker Company, Oakland,

Calif.

22 Filed: May l, 1972 [21] Appl. No.: 249,395

[52] US. Cl 175/5, 175/27, 173/4, 254/172 [51] Int. Cl E2lb 19/08 [58] Field of Search 175/5, 7, 27, 85; 173/4, 173/6; 254/172, 188; 214/25; 294/82 [56] References Cited UNITED STATES PATENTS 2,613,102 10/1952 Roberson 214/25 X 3,208,728 9/1965 Parks 8/1966 Homanick 214/25 X 3,311,351 3/1967 Blakely 254/172 3,706,347 12/1972 Brown 3,718,316 2/1973 Larraldlc et al. 175/5 X Primary Examiner-Marvin A. Champion Assistant Examiner-Richard E. Favreau Attorney, Agent, or Firm-Lothrop & West [5 7] ABSTRACT For use with a standard drilling rig on a floating barge, there is interposed between the travelling block frame and the drill string support a drill string compensator having a cylinder and piston expansible chamber (with or without a motion multiplier) furnished with air under pressure within a normally closed volume. The

air chamber acts as a cushion and is arranged so that the variation in chamber volume produces only a small change in the air pressure, which can be set at any desired average value. Motion of the pneumatic, expansible chamber is controlled by a hydraulic expansible chamber. Various emergency and manual controls are provided. 1

8 Claims, 7 Drawing Figures PATENTEUAPR 16 19M 3 804' 183 PATENTEDAPR161974 I 3.804.183

SHEET 3 (if 5 IIIII "LIZ iii-3w DRILL STRING COMPENSATOR In the drilling of wells beneath the surface of the water it is customary to support a standard drilling rig on a barge floating on the water and to conduct the drilling operations in the usual fashion as nearly as may be. Unfortunately, the operation of the drilling rig is greatly upset by any major rise and fall of the supporting barge, and many expedients have been resorted to in order to alleviate or obviate the difficulties thereby induced.

Without considering in detail the difficulties and deficiencies of many of the arrangements provided for overcoming the resulting troubles, it is in general an object of the invention to provide a greatly improved and more nearly satisfactory drill string compensator.

Another object of the invention is to provide a drill string compensator that employs pneumatic fluid for a principal portion of its work, subject, however, to rigid control by hydraulic mechanism.

A further object of the invention is to provide a drill string compensator which will fully or partially compensate for the heaving or rising and falling of the drill string supporting vessel.

Another object of the invention is to provid'ea 'drill string compensator which is quite efficient in its utilization of energy, can be readily incorporated in drill strings of standard construction and is a self-contained unit not requiring substantial revision of drill string equipment already available.

A further object of the invention is to provide a drill string compensator operating almost completely automatically, if desired, or permitting the use of manual control. l

Another object of the invention is to provide a drill string compensator that generally is fail-safe.

A further object of the invention is to provide a drill string compensator allowing for a large vertical excursion of the supporting vessel without substantial vertical movement of the drill string and without substantial change in force on the drill bit.

Another object of the invention is to provide a drill string compensator allowing for a large vertical excursion of'the supporting vessel without excessive movement of the compensator parts.

Another object is in general to provide an improved drill string compensator. 7

Other objects together with the foregoing are at tained in the embodiments of the invention described in the accompanying description and illustrated in the accompanying drawings, in which:

FIG. I is a diagrammatic view showing a floating barge supporting a drilling mechanism with a compensator included therein;

FIG. 2 is a diagrammatic showing, in cross-section. of one form of drill string compensator pursuant to the invention. the control instrumentalities being omitted;

FIG. 3 is a cross-sectionshowing diagrammatically some of the control instrumentalities as they are'incorporated in the FIG. Zstru'cture;

FIG. 4 is an isometric view, with portions being broken away. of part of a drill string compensator pursuant to the invention;

FIG. 5 is a cross-section showing diagrammatically some of the control instrumentalities as they' are incorporated in the FIG. 4 structure;

FIG. 6 is a diagram showing the interrelationship and connections of one form of drill string compensator pursuant to the invention; and

FIG. 7 is a diagram somewhat like FIG. 6 but showing a different form of drill string compensator pursuant to the invention.

In a typical environment a well 6 is drilled by a rotary drill 7 in the earth 8 below the surface 9 of a body of water 11. The water body is subject to wave action, which may have a vertical magnitude illustrated by the double-headed arrow 12. Floating on the water body is a barge 13 supporting a super-structure 14. A main hoist 16 and an anchor 17 on the super-structure between them carry a support line 18 trained around a travelling block 19. Arranged below the travelling block is a drill string support 21 provided with a hook 22. Hanging from thehook is a swivel 23 from which the drill pipe 24 is suspended through a rotary table 26 in the usual way. The drill pipe 24 can be lifted and lowered so as to lift and lower the drill 7. The drill pipe is rotated by the table 26 as indicated by the arrow 27 in FIG. 1.

Customarily, the travelling block 19 is directly connected to the support 21 or to the hook 22. If this custom is followed, then as the barge l3 rises an falls due to movement of the water 11 the drill 7 is correspondingly lifted and lowered. This is highly disadvantageous and sometimes compensating mechanisms are provided in the drill string within the hole 6.

In accordance with the presen't'invention, however, a different arrangement is afforded. Interposed between the travelling block 19 and the support 21 is an expansible chamber In the present instance the chamber is preferably duplicated so that a similar chamber 32 is provided symmetrically with regard to the center line of the drilling rig. Each of the chambers 31 and 32 is defined by a cylinder 33 within which a piston or plunger 34 is vertically reciprocable. Conveniently, the upper end of each cylinder is open to pass its plunger or piston rod. which at the upper end carries a pulley 36 over which achain 37 is passed. The chain is in effect a motionmultiplier. At one end each chain is dead-ended against its cylinder 33 and at the other end passes down to and is fastened onto the support 21.

The two cylinders 33 are connected in parallel by lines 38 and 39 with a source 41 of gas, such as an air tank, under pressure. The volume of the connected tank 41 and the cylinders 33 together with the interposed piping is relatively large. The motion of the pistons 34 up and down as the barge l3 rises and falls, although adequate to care for the expected travel 12, is nevertheless productive of only a minor pressure fluctuation in the substantially closed pneumatic system. In effect thereis provided a pneumatic cushion supporting the drill 7 with approximately the same force despite motion of the barge.

The average pressure supporting the drill can be a maximum to keep the drill virtually floating in place or can be reduced to some smaller value to allow any desired gravity force on the drill. Furthermore, the'pressure within the pneumatic system can be great enough so that the drill 7 can be actually raised to a limited extent.

Because this system is preferably at least partly pneumatic, the gas interflow pressure loss between the variable volume or expansible chambers 31 and 32 and the pipelines to the rest of the gas system, particularly the tank 41 or air pressure vessel, is relatively small. The amount of power consumed by the cushioning or compensating system is correspondingly small.

Although not shown in FIG. 1, but apparent from some of the other figures, there is also provided a liquid or hydraulic control system for the pneumatic circuitry so that the motions of the parts are precisely as designed and as desired. The general nature of one form of the hydraulic mechanisms is set forth in FIGS. 2 and 3, in which a simplified version of the structure is diagrammatically disclosed. The travelling block 19 is secured to a block frame 51, to which cylinders 52 and 53 (like the chambers 31 and 32) are stationarily secured. Operating within the cylinders are pistons 54 and 56 at the end of piston rods 57 and 58 acting in tension to suspend a drill string-support 59 (like the support 21) to which the hook 22 is fastened. Each of the cylinders 52 and 53 is connected through a velocity control, a restricted orifice 61, with one of a pair of reservoir chambers 62 containing a body 63 of oil in the lower portion and having an enclosed volume 64 for air or other suitable gas in the upper portion. To provide the benefits of oil lubrication of the cylinders 52 and 53 and air coupled flow piping between the cylinders 52 and 53 and the air pressure vessel 41 (FIG. 1), the airoil reservoir chambers 62 are mounted immediately on the cylinders 52 and 53; The upper volumes64 are connected throughlines 66 having control valves 67 therein to a source of air or other gas under appropriate pressure (not shown).

In this elementary form of arrangement, as the travelling block 19 rises and falls with the barge 13, it moves vertically relative to the piston rods 57 and 58 and to the hook 22. This relative motion causes flow of oil through the restricted orifices 61 into and from the chambers 62. Air pressure in the volumes 64 is maintained at a high enough value substantially tosustain the load on the hook 22. Although not to scale in the diagrammatic figures, the volume 64 is preferably many times the volume of displacement of the pistons inside the cylinders. There is thus only a small air pressure change for the full stroke of the pistons. In effect the hook 22 tends to be supported by a fairly constant, predetermined. upward elastic fo'rce' no matter what the vertical position of the pistons in the cylinders.

The hydraulic flow is especially governed so that the motion of the parts is as designed. The particular arrangement preferred in the type of device illustrated in FIG. 2 is shown in more detail in FIG. 3. The piston 54 is provided with a cup 68 sustaining a shallow layer 69 of oil and closely telescoping with a boss 70 depending from the cylinder head. In the event load is abruptly re moved from the piston rod 57 and the piston 54 rises quickly, its travel is braked and slowed in the upper portion by the restricted flow of oil between the cup 68 and the boss 70. Somewhat similarly, motionof the piston downwardly is restricted in the bottom portion of the travel by entry of a boss 71, preferably tapered, into a well 72 of reduced diameter disposed in the oil body 73 between the cylinder proper and the orifice 61. While the orifice 61 limits the piston velocity under normal conditions, the bosses 70 and 71 sharply limit that velocity at either end of the stroke.

In a practical example, as shown in FIG. 4, the travelling block 19 is supported by multi-part lines 74 in the usual fashion. In this instance, particularly, the block frame 51 is made up of side plates 75 and 76 between 4 them supporting cylinders 77 and 78 on opposite sides of the center line. These correspond to the cylinders 52 and 53. Projecting from the cylinders are plungers 79 and 80 operating in compression and at their upper ends carrying pulley blocks 81 and 82 over which are reeved multi-part chains 83 and 84. The chains at one end are secured by anchors 85 to the plates 75 and 76 on the block frame 51 and at their other end are secured to the drill string support 59 (21 in FIG. 1) from which the hook 22 depends. In FIG. 4 substantially .standard parts are shown in broken lines and the compensator is illustrated in full lines.

Each of the cylinders 77 and 78 is provided with flexible connecting means 86 extending to a block 87 at some convenient, stationary part of the structure. A pipe 88 extends from the hose structure 86 to the pneumatic reservoir or tank 41. With the arrangement as described, it is relatively simple to disconnect a standard travelling block 19 from its usual direct junction with the support 59 and hook 22 and to interpose between the disconnected block 19 and drill string support 59 a block frame 51 and appurtenant parts in order to introduce the drill string compensator into a standard arrangement.

In a typical, complete installation, as schematically shown in FIG. 6, many of the parts remain as particularly shown in FIG. 4. In both cases, the piston rods or plungers 79 and 80 are utilized in compression, as distinguished from the simplified arrangement shown in FIG. 2, in which the piston rods 57 and 58 are operated in tension.

The pneumatic operating fluid is preferably a gas and. while a relatively exotic, inert gas can be used, it

is found in practice that ordinary atmospheric air prop.- erly compressed and dried is an effective medium. For reliability and ease of installation there is provided a pair of driving motors 91 and 92 operating a pair of air compressors 93 and 94 with their accompanying standard drying mechanisms. Air under a predetermined pressure is supplied through check valves 95 to a conduit 96 leading to a distribution network.

Within this network the conduit 96 has a branch 97 extending to a pressure switch 98, so that in the event of over-pressure the m'otors9l and 92 are shut down. Also within the network the conduit 96 extends through a check valve 99 to a control valve 100. This is preferably remotely controlled and is set to produce an increase in pressure of the compressed air to any predetermined, reasonable value.

From the valve 100 the conduit 96 extends through a main control and shut-off valve 101 that is preferably manually regulated and is utilized in putting the system into operation and in shutting it down. The conduit 96 extends to a junction point 102, at which a pressure gauge 103 controlled by a valve 104 is disposed to indicate the system operating pressure. From the junction point 102 there is a branch conduit 105 extending through a manually controlled charging valve 106 to an extension conduit 107 which has, if desired, a quickdisconnect joint 108 therein. The pressure within the conduit 107 is indicated by a gauge 109 under the control of a valve 110. The conduit can be bled to the atmosphere, if desired, through a valve 111.

The extended conduit 107 branches into a number of connectors 112, each being like the others, and all being connected in parallel. Each connector 112 extends through a valve 113 into a reservoir or pressure tank 114. A drain valve 116 is provided on each tank as well as a pressure relief valve 117. In this case there are six, parallel, pressure tanks for storing a volume of air under relatively high pressure. The air supply is available at the junction point 102 not only from the compressors 93 and 94 but, after they have been charged, also from the tanks .114.

It is preferred also to provide some'emergency or standby pressure air capacity. For that reason from the conduit 97 flow is through a check valve 118 and a,

manual valve 119 into branch conduits 122 and 123 leading to auxiliary air consumers, not pertinent to the rest of this disclosure. The valve 119 also opens into a conduit 124 controlled by a valve 126 and leading to a conduit 127 having a junction point 128. From the junction point 128 a line 129 extends to a junction point 131. In one direction from the junction point 131 a line 132 extends to a single pressure tank 133 just like the accumulators 114 and, like them, supplied with an entrance valve 134, a drain valve 136 and a pressure relief valve 137. The pressure in line 129 is indicated by a gauge 138 under control of a hand valve 139. The line 129 has a vent valve 141 for blowing down or drainage.

With this arrangement the standby pressure tank 133 is normally maintained at the same pressure as the tanks 114 and, while it does not have as great volumetric capacity, it is sufficiently large to serve on a standby basis. If desired, the junction point 131 can serve as a connector to a line 142 extending. to an indicator 143 at a remote point, preferably a distant control station, where the various operations of the structure are indicated and are controlled or manually set. Pressure within the standby system can be made available from the junction point 128 through a check valve 144 and into a pressure increase valve 146 to a junction point 147. 'Preferably the pressure increase valve 146 is shunted by a line 148 having a remotely operated con.- trol valve 149 therein, so that in any event the junction point 147 as well as the junction point 102 can, when desired, be locally provided with standby or. auxiliary air. 1

Air under pressure from the points 147 and 102 is carried by the conduit 96 through a control valve 151. On the downstream side of this valve there is a connecting line 152 leading to' an automatic pressure decrease valve 153 as well as to a manual pressure decrease valve 154, the pressure being indicated by a gauge 156 controlled by a valve 157. Also downstream of the valve 151 there is a quick-disconnect coupling 158 going to a conduit 159 having branches 161 and 162.

Each ofthe lines 161 and 162 is provided with a valve 163 leading through a check valve 164 to a line 166. This is joined to a pair of flexible hoses 167 leading to a duct 1168 provided with a blow-down valve. 169. The

lines 168 extend through check valves 170 and shut-off valves 171. One of the valves 171 opens to a conduit 172 in which a blow-down valve 173 is situated. The conduit 172 also has a pressure relief valve 174 and joins the cylinder 77 beneath a piston 176 therein disposed at the lower 'end of the plunger rod 79. In this fashion air under pressure is supplied to the cylinder 77. Similarly, a branch conduit 177 .from the other valve 171 and from the conduit '1'72'extends to the cylinder 78 beneath a piston 178 disposed at the lower end of the plunger rod 80. In this fashion the cylinder 78 is subject to the same internal pressure as is the cylinder 77.

Since gas is an elastic medium and cannot function to control the exact motions and locations of the pistons 176 and 178, there is provided a hydraulic means for that purpose. To the cylinder 77 above the piston 176 and from the volume 181 thereabove a line'182 extends through a special control valve 183 to a dipper pipe 184 ending below the surface 186 of a body 187 of oil within a pressure vessel 188 having a drain valve 189 thereon. The valve 183 includes a check valve which opens for flow out of the vessel 188 and which closes to prevent flow back into the vessel. There is a shunt path 191 around the check valve. 185 under control of a variable orifice 192. Flow of liquid out of the vessel 188 through the open check valve occurs at a 7 its inflow through an adjustable orifice 208 to a desired rate.

As especiallyshown in FIG. 5, means are provided for limiting travel of the piston 176 at both ends of the cylinder 77. A piston boss 209 can enter a restricted well 210 beneath an oil pool 211 at the cylinder bottom, thus acting as a hydraulic brake. A piston boss 212, preferably tapered, can enter a restricted volume 213 at the upper end of the cylinder77 and so act as a hydraulic brake at that end.

The space above the oil body in each of the vessels 188 and 206 is maintained at a predetermined pressure. From the conduit 177 flow is through a restricted orifree 214 and a pressure regulating valve 215 to a junction point 216, at which a pressure gauge 217 is disposed under the control of a valve 218. From the junction point 216 a line 219 extends through a manual control valve 220 to the upper part of the vessel 206.

A pressure relief-valve 221 makes sure that thegas pressure within the upper portion of the-vessel 206 is not excessive. From the junction point 216- a similar line 222 extends through a hand control valve 223 to the upper portion of the vessel 188. Pressure in this upper portion is kept below a predetermined maximum by a gas pressure relief valve 224.

In the initial charging of the system, which may occur before there is any substantial load on the hook 22, it is desirable to prevent excessive pressure coming onto the lifting pistons 176 and 178 without some load imposed thereon; otherwise they might be lifted to an extreme position at an improper time. The parallel check valves 164 close in the charging direction so that when air pressure within the cylinders 77 and 78 is relatively low, incoming high pressure air cannot then pass the check valves 164. It is preferred that the downstream part of the system be slowly charged with high pressure air before the check valves are opened. To that end there is provided at the junction point 147 a conduit 226 leading through a remotely operated control valve 227 to a conduit 228 having a quick-disconnect coupling 229 therein. This-is joined to a conduit 231 having branches 232 and 233. Each of these branches has therein a downstream opening check valve 234 leading through a restricted orifice 236 into the related pipe 166.

When the system is being charged, the valve 227 is opened and, as regulated by the orifices 236, the downstream portion of the system is charged until its pressure is virtually at an operating value. Then, when the main supply of air is furnished. the pressure on the opposite sides of the check valves 164 has been virtually equalized and they are held from their seats by air from the lines 232 and 233 acting upon them through connecting lines 237. I

Should there be an accidental loss of the main load on the hook 22, the pressure beneath the pistons 176 and 178 would normally tend to lift the support 59 at an excessive rate. However, the hydraulic fluid above the pistons 176 and 178 can escape through the conduits 182 and 202 back to the vessels 188 and 206 only at a rate regulated by the adjustable throttling orifices 192 and 208. Thus there is a hydraulic retarding or brake effect which precludes rising of the support 59 at an excessive rate.

Although not part of the main system. there are conveniently provided remotely controlled air lines 238 and 239 extending to a control cylinder 241. Through a (diagrammatically illustrated) rod 242 the cylinder governs the motion of the hook 22. The load on the hook can be captured or released, as desired.

In a somewhat simplified version of approximately the same general system, there is shown in FIG. 7 an arrangement in which an electric motor 251 drives an air compressor and dryer 252 supplying air through a conduit 253 containing a check valve 254 and through a second check valve 256 to a junction point 257. Pressure within the line 253 is effective upon an electrical switch 258 controlling the operation of the motor 251. From the junction point 257 a line 259 extends through a compressor shut-off valve 261 which can be utilized to isolate the compressor 252. The line 259 extends through a valve 262 to a gauge 263 for indicating line gas pressure. The line 259 has a junction point 264 from which a line 266 extends through a connecting duct 267 toa vent valve 268 leading to the atmosphere. The line 266 also extends through a charging valve 269 and a conduit 271 and connects through shut-off valves 272 to a bank of pressure vessels or tanks 273. Each tank has its own drain valve 274 and its own pressure relief valve 276'.

In a comparable fashion, a duplicate electric motor 277 drives an air compressor and dryer 278 discharging through a check valve.279 into a line 281 leading to a second check valve 282. Joined to the line 281 is a pressure switch-283 governing the operation of the motor 277. Downstream of the check valve 282 there I is a compressor shut-off valve 284 so that the compressor 278 can be isolated from mechanism downstream thereof. From the valve 284 a connector 286 extends to a junction point 287 with the line 266.'From the other side of the junction point 287 a line 288 has a branch 289 extending through a vent valve 291 to the atmosphere. The line 288 also has a control valve 292 connected by a line 293 and appropriate shut-off valves 294 to a bank of pressure vessels or tanks 296, each with its drain valve 297 and pressure relief valve 298. There is thus afforded a redundant or duplicate air supply and storage complex.

Extending from the line 266 is a line 301 regulated by a control valve 302 for activating the compensator. The line 301 goes to a shut-off valve 303 leading to a check valve 304 closing in the downstream direction. Connection is then through duplex flexible conduits 305 and through a downstream opening check valve 306. Connected upstream of the check valve 306 is a remotely controlled pressure relief valve 307, while downstream of the check valve 306 is a shut-off valve 308 opening into a line 309 extending to the bottom of the cylinder 78, as before.

Ina similar fashion, from the line 288 and through a valve 311 extends a conduit 312 connected through a shut-off valve 313 and a downstream closing check valve 314 to a pair of flexible ducts 316 extending to a line 317. There is a cross connection 318 from the line 317 to the relief valve 307. A similarline 319 connects downstream of the valve 306. On the downstream 'side the line 317 has a downstream opening ball check valve 321 leading through a shut-off valve 322 to the line 309. Paralleling the line 309 is a line 323 extending to the cylinder 77, with the remaining connections being substantially as previously described.

There is afforded a means for opening the downstream closing check valves 314 and 304 upon starting. From the junction 257 a line 326 extends through a control valve 327 into a duct 328 which has branches 329 and 331. Each branch-is provided with a downstream opening check valve 332 and leads to a related one of a pair of conduits 333 and 334. Air pressure in the conduits 333 and-334 is effective to hold open the check valves 304 and 314 from their seats.

As an additional feature, a hook latching and un latching mechanism 336 includes a separate fluid operating' system 337 which through a control valve 338 regulates the motion of a cylinder and piston structure 339 for operating the hook closure through an intervening (diagrammatically shown) rod 341.

This layout provides a remotely controlled mechanism for accomplishing'in a simplified fashion substanone. end of said chain to said block frame, meansfor.

fastening the other end of said chain to said drill string support, and means for varying the pneumatic pressure in said expansible chamber.

2. A device as in claim 1 in which said expansible chamber includes a cylinder and a piston and said cylinder is mounted vertically on said block frame and said piston extends vertically into the upper end of said cylinder I 3. A device as in claim 2 including a pulley mounted at the upper end of said piston, a chain trained over said pulley, means for fastening one end of said chain to said block frame, and means for fastening the other end of said chain to said drill string support.

near the ends of said stroke for arresting said piston.

7. A device as in claim 1 in which the volume change of said expansible chamber is a predetermined amount and a pneumatic pressure tank having a volume that is a selected multiple of said predetermined amount is connected to said expansible chamber.

8. A device as in claim 7 in which said pneumatic pressure tank is mounted on said expansible chamber. 

1. A drill string compensator comprising a travelling block, a drill string support adapted to be suspended from said block, a block frame secured to said travelling block, a pneumatic pressure responsive expansible chamber having relatively movable members, means for connecting one of said members to said block frame, a pulley mounted on the other of said members, a chain trained over said Pulley, means for fastening one end of said chain to said block frame, means for fastening the other end of said chain to said drill string support, and means for varying the pneumatic pressure in said expansible chamber.
 2. A device as in claim 1 in which said expansible chamber includes a cylinder and a piston and said cylinder is mounted vertically on said block frame and said piston extends vertically into the upper end of said cylinder
 3. A device as in claim 2 including a pulley mounted at the upper end of said piston, a chain trained over said pulley, means for fastening one end of said chain to said block frame, and means for fastening the other end of said chain to said drill string support.
 4. A device as in claim 1 including hydraulic means employing said chamber in part for damping the relative movement of said members.
 5. A device as in claim 4 in which said hydraulic means includes a hydraulic flow limiting means effective to limit the rate of upward movement of said drill string support.
 6. A device as in claim 4 in which said hydraulic means includes a piston operating in said cylinder through a predetermined stroke, and means effective near the ends of said stroke for arresting said piston.
 7. A device as in claim 1 in which the volume change of said expansible chamber is a predetermined amount and a pneumatic pressure tank having a volume that is a selected multiple of said predetermined amount is connected to said expansible chamber.
 8. A device as in claim 7 in which said pneumatic pressure tank is mounted on said expansible chamber. 